Electrical circuit and transformer therefor



Feb. 3, 1931.

F. H. DRAKE ELECTRICAL CIRCUIT AND TRANSFORIER THEREFOR Filed llay'4. 1928 {Shays-Sheet 1 4 J 1 a v p 6 5 1 z.

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F. H. DRAKE ELECTRICAL CIRCUIT AND TRANSFORMER-THEREFOR Filed May 4.1928 2 Sheets-Sheet 2 gmnntdt Patented Feb. 3, 1931 UNITED STATES PATENTOFFICE FREERICK H. DRAKE, OF BOONTON, NEW JERSEY, ASSIGNOR TO RADIOFREQUENCY LABORATORIES, INCORPORATED, OF BOONTON, NEW JERSEY, ACORPORATION OF NEW JERSEY ELECTRICAL CIRCUIT ANE TRANSFORMER THEREFORApplication. filed May 4,

. This invention relates to electrical systems 01' circuits of which atypical example, with reference to which the invention will hereafter hedescribed, is an electrical amplifier circuit of the type employing oneor more electron or vacuum tubes such as are commonly known as audions,triodes, tetrodes, and the like, and suitable for use in radio receivingsets. It relates also to an electrical transformer suitable for use insuch a circuit or system, for example as a. coupling means between tubesof the foregoing types, and in particular, also, to a radio frequencytransformer proportioned, designed, and constructed according to a novelmanner, and which is suitable for use in such electrica circuits orsystems.

In electrical amplifiers of the foregoing type, especially in tunedradio frequency amplifiers for example, it is frequently desirable toemploy coupling means or coils of the socalled auto-transformer type;that is, a single coil one portion of which constitutes the input coilof the transformer, and another portion of which, perhaps including thefirst portion, acts as the output coil of the transformer. An advantageof such an auto-transformer coil is that the impedances of the portionsthereof may be readily adjusted according to inputand outputrequirements. For example, in the case of certain tubes, the input andoutput impedances of the appropriate portions of the transformer may beapproximately adjusted or equated to the tube input and outputimpedances with which they are associated, in order to produce highamplification, or the input impedance of the transformer may be madelower than the output impedance of the tube with which it is associated,so as to improve selectivity. More- I over, by using suchauto-transformer coils it is possible to obtain a very high degree ofcoupling between the input and output circuits, for example by using a1: 1 turn ratio, without introducing such undesirable capacity andresistance effects in the secondary as may be entailed by the use ofconventional transformers having separate primary and secondary coils.For example, such an autotransformer is highly desirable for use inSerial No. 275,177.

connection with a radio frequency amplifier employing electron tubes ofthe type having transformers in association with electron tubes of thetypes referred to, a number of difficulties are commonly encountered.For example, it is difficult or impossible to use common B-batteries, ora common socketpower source of B supply, in multi-stage amplifiershaving auto-transformer coupling, without the use also of additionalpieces of apparatus. Since in almost all practical multi-stageelectrical amplifiers it is necessary or desirable to use common sourcesof direct current potential for the anodes of the several stages,various ways of accomplishing this'purpose in auto-transformer coupledamplifiers have been attempted. F or example, in order to isolate thepotential of the anode of a tube from the grid of a succeeding tube, ithas been proposed to use a blocking condenser inserted between the highpotential secondary terminal of the coupling transformer and the controlelement of the succeeding tube, a grid biasing resistance between thegrid and filament of this tube being then also required. Another methodwhich has been proposed is to insert a blocking condenser between theplate'of the first tube and the high potential primary terminal of theauto-transformer, direct current being then fed to the plate through asuitable radio frequency choke coil. Neither of these methods ofemploying auto-transformer coupling between cascaded tubes in amulti-stage amplifier having common sources of direct cura circuithaving all of the advantages and electrical simplicity of anauto-transformer coupled circuit in addition to the advantage of beingable to employ common sources of direct current potential for the anodesof the several stages, Without the disadvantages of -the systems abovereferred to.

I will now describe my invention in connection with the accompanyingdrawings in which Figure 1 represents a multi-stage electrical amplifiersystem including two high impedance tubes of the tetrode type arrangedincascade according to my invention; Figure 2 illustrates my inventionas applied to a triode tube arranged in an electrical amplifiercircuitof the balanced type; Figure 3 illustrates diagrammatically atransformer according to my invention; and Figures 4 and 5 are specificexamples of structural details of such transformers arranged accordingto my invention.

Referring now to Figure 1 the tube VT is the first tube of a multi-stageelectrical amplifier and is provided with anode or plate, P, controlelement or grid G electron emitting element or cathode, F, and a fourthelectrode in the form of a second grid G The input voltage E0 is appliedbetween the input terminals 1 and 2 connected to the control grid andground, or between the control grid and cathode if a C-battery isemployed a as the grid biasing means. A transformer T has a winding Ltuned by variable condenser C and connected between the control grid G.and cathode F of the second tube VT of the multi-stage amplifier. Thewinding L consists of wire of gauge relatively large compared to thegauge used for L and is associated and coupled by mutual inductance Mwith a twin winding, L consisting of wire of small gauge. This windingmay be wound from the cathode or ground terminal of the coil L to anydesired distance toward the high potential terminal thereof. Thetransformer consists, accordingly, of a winding L of relatively largewire and a winding L of relatively small wire closely associated andcoupled with a partor all of the winding L and forming a twin Windingtherewith. The low potential terminal of the winding L is connectedthrough the B-battery or equivalent source of direct current potentialto the cathode F, and the lead 5 from the anode, P, is-connected to asuitable point in the winding L or may be connected toa variable tap onthis winding as shown- The relatively small winding L thus serves tosupply directcurrent potential to the anode or plate of the first orinput tube VT and is accordingly herein designated as the potentialwinding. The winding L of relatively large wire carries the mainresonant current in the tuned secondary circuit 11 -6 and is thereforeherein termed' the current winding cuit including a tube VT The secondtube, VT of the multi-stage amplifier may, similar transformer T havingan input or potential winding L and anoutput or current winding L Thewinding L may be tuned by variable condenser O as before, the outputvoltage between the output terminals 3 and f being designated as E. Anynumber of amplifier stages may be cascaded in such a multi-stageamplifier, and the last of these may suitably work into a detectorstage, or whatever other load may be desired. When such an amplifier isemployed in a radio receivingset, the terminals 1 and 2 ma be theantenna and ground termina s, and the terminals 3 and 4 may be connectedto the detector. It will be seen that the cathodes of the several tubesare connected to a common A-battery, or other source of direct current(a battery being shown by way of example) or the cathodes may assume theform of alternating current filaments or may be of the indirectly heatedtype, in which latter cases the grid bias may be sup lied by well knownmethods. A common %3-battery, or equivalent source of direct currentpotential, may be connected between the terminals A and Ep; and thegrids Gr may be given a' suitable potential by an appropriate connectionof the terminal E as, for example, by a connection to the B-battery. Oneor, more by-pass con-. densersC'may be associated with the foregoingconnections to. the appropriate current sources for example as indicatedin the drawing. The grids G may be biased negatively with respect to thecathodes by means of resistance R or by other well known means arheostat, B, may also be included in the cathode circuit if desired.

Figure 2 shows an of the triode type provided with anode or plate, P,control element or grid, G, and electron emitting cathode or filament,F. The input of the amplifier is connected to the terminals 1 and 2. Atransformer T is associated with tube VT in the amplifier, and comprisesa windif desired, be supplied with a electrical amplifier ciring L of.gaugerelatively small compared with the gauge employed for the windingL The winding L forms a twin winding with a suitable portion of thewinding L and is arranged to supply direct current potential to theanode P. Its lower terminal is connected to the cathode through a B-battery or equivalent direct current source, as shown. The winding L maybe tuned by a variable condenser C A tube of the triode type ordinarilyhas inter-electrode capacities, particularly a capacity between anodeand control element, which are apt to produce disturbing feedbackeffects in an amplifier circuit with which the tube is associated. Theseintermented by stray external capacities between leads, etc. In order toreduce or eliminate the flow of regenerative or retroactive currents dueto this capacitive coupling between the output and input circuits of theamplifier, it has heretofore been proposed to employ a balancing circuitarranged to feed back to the input circuit additional currents of theproper phase and amplitude to oppose undesired retroactive currents.According to my invention I may employ a balancing winding L connectedto the cathode, and

coupled to the windings L and L of the transformer in the reverse sense,or having its connections reversed, so that the mutual inductances M andM are negative in sign. This balancing winding I. may then appropriatelybe connected to the grid G through a balancing condenser C the wind ingL mutual inductances M- and M and capacity C being so proportioned andarranged as to feed back on to the grid O currents of the proper phaseand amplitude to reduce or suppress the effects of currents flowingthrough undesired capacitive coupling between the output and inputcircuits. I prefer to employ the additional balancing winding L ratherthan to reverse the connections to L and connect C to a point on L sincein the latter arrangement the capacity between the windings L and L actsto produce undesirable eifects.

Figure 3 illustrates in detail a transformer arranged according to myinvention, the coil L consistin of relatively large wire, and the coil Lbelng wound as a twin-winding withcoil L and consisting of wireconspicuously smaller in gauge than that employed in the winding L L iswound so as to form a twin winding with so much of the winding L as isdesired. The winding L may therefore extend 'a part or all of the wayalong the winding L The wire forming the winding L is made relativelylarge because it carries the main resonant current of the tuned circuitL C and thewire forming the Winding L is preferably conspicuouslysmaller in gauge, since its function is merely to supply direct currentpotential to the anode of the input tube, and to form acoupling to themain currentcarrying winding L The input and output connections to thesewindings may be arranged as indicated. Atwin-windingasthe term is hereinemployed, may be defined as one where the primary winding L is woundturn by turn in the same sense along and with the corresponding turns ofthe secondary winding as shown; so that if L IS the inductance of theportion of the secondary winding along which the primary extends, L isthe inductance of the primary winding, and M is the-mutual inductancebetween L and L then L and L are approximately equalftgpeach other andto M Figures 4 and 5 are cross-sections of typical examples oftransformer structures embodying the invention as above described. InFigure 4 the turns of the winding L are interposed between those of thewinding L the two wires being wound side by side upon a suitable form 6,and constituting only a single layer. In Figure 5 the fine wire L iswound in a groove 7, which may advantageously be of V shape, in the form6, while the current windingL is wound over the winding L in the samegroove.

-The use according to my invention of a potential or input winding ofrelatively or conspicuously smaller gauge wire than the wire employedfor the current winding associated with the resonant circuit isdesirable in order that the power factor and physical dimensions of thetransformer may be kept down to values approximating those of a simpleauto-transformer. For example, with a current winding of #28 enamel wireI have found, in a specific case, that an input or potential winding of#36 enamel wire is suitable. In a construction similar to that shown inFigure 4, the resultant power factor of the current winding, in onespecific physical transformer, was found to be increased by less thanten per cent over its value when no potential or input winding waspresent. On the other hand, when the input winding is of a gauge near tothat of the current winding, the power factor of the current winding isfound to be increased by many times this amount, this effect being dueboth to the presence of additional conductor in the field and to thefact that the form factor of the coil is necessarily much inferior forthe same diameter of coil.

A twin winding of the type described above is found to be superior tothe ordinary type of primary or input winding in that capaclty betweenany turn of the potential winding and the corresponding secondary turninvolves no current flow, due to the fact that the two turns are at thesame alternating current potential. The effective capacity of thesecondary of such a twin winding transformer is therefore less than inthe case of an ordinary transformer having primary and secondarywindings on separate forms, unless in such a case the primary winding ismade of considerably smaller diameter than the secondary winding. In thelatter case I described above, the windings consisting of wire ofconspicuously different gauges, I may give the following constants whichI have successfully employed in a transformer used in connection. with acommercial tetrode: i

L =85 turns #36 enamel on 1 form L- .=85 turns #28 enamel on 1 form\Vhen measured in a round copper shield 3" long x 3 in diameter, thistransformer showed the following radio frequency resistance values (Rfor the winding L At 550 meters R =L1 ohms At 300 meters R =6.8 ohms\Vith the winding L entirely removed, the corresponding values for theradio frequency resistance of L were At 550 meters R:=3.9 ohms At 300meters 1t =62 ohms These figures show that the use of the relativelyfine wire primary gives a transformer of substantially as high merit asa simple auto-transformer, it having been determined that the inductanceof L is not sensibly altered by removal of thefine winding.

Another specific example is as follows:

L =85 turns #34 enamel wire on 1 form v L =85 turns #28 enamel wire on 1form Measured in the'same way the radio frequency resistances were:

At 550 meters R 4.8 ohms At 300 meters R ==7A ohms I With the finewinding L removed the corresponding resistances are:

At 550 meters R =4A ohms I At 300 meters R ==6A ohms From a comparisonof the foregoing figures showing the relative resistances of thetransformer secondary for the two sizes of primary wire, the advantageof a substantial or conspicuous difierence in the size of wire used forthe input and output windings of the transformer is apparent. As thesize of the wire in the primary winding approaches the size of the wirein the secondary winding, the secondary resistance increases, and thisincrease is due to at least two factors, namely (1) because the coilwithout the primary has a poorer form factor due to the increasedspacing of the secondary turns to accommodate the primary turns and 2)because the amount of conductor near t e secondary winding is increasedand the eddycurrent losses are consequently larger.

.The foregoing details of constructions and constants are merelyspecific examples according to my invention, which is not limitedthereto. Moreover, its application is not limited to electrical circuitsof the type illustrated, these being chosen merely as examples of themany applications of my invention.

Many variations and modifications of the specific circuit arrangementsand transformer v transformer constructions described herein, fallwithin my invention.

I claim: 7

1. An electrical amplifier system, comprising, in combination, a tubehaving at least an anode, a cathode, and acontrol element; a

having a current winding consisting of relatively large wire and apotential winding consisting of relatively small wire wound turn by turnin the same sense along and with corresponding turns of at least a partof said current winding to form a twin Winding therewith; and meansassociated with said potential winding and said tube and adapted topermit the application of a direct current potential between the anodeand cathode of saidtube.

An electrical amplifier system, comprising, in combination, a tubehaving at least an anode, a cathode, and a control element; a

transformer having a current winding consisting of relatively large wireand a potential winding consisting of relatively small wire wound turnby turn in the same sense along and with corresponding turns of at leasta part of said current .winding to form a twin winding therewith; asource of direct currentpotential; and connections including saidpotential winding for applying said direct current potential between theanode and cathode of said tube.

3. A multi-stage electrical amplifier comprising, in combination, a tubehaving at least an anode, a cathode, and a control element; atransformer having a current winding consisting of relatively large wireand apotential winding consisting of relatively small wire wound turn byturn in the same sense along and with corresponding turns of at least apart of said current winding to form a twin winding therewith; aconnection from the cathode of said tube through a source of directcurrent potential to said potentiall winding, and a connection from saidpotential winding to the anode of said tube; a tunof said currentwinding to form a tuned circuit; and a second tube having its inputterminals associated with and fed from "said tuned circuit; 7

4. A inulti-stage electrical amplifier circuit comprising, incombination, a pluralityof tubes each having at least an anode, acathode, and a control element; an electrical coupling system interposedbetween two of said tubes and including a transformer having a w1rewound turn by turn in:

mg. condenser connected across the terminals between the anode and thecathode of each of said tubes, the connection to the anode of the firstof said tubes including said potential winding.

5. A multistage electrical amplifier comprising, in combination, a tubehaving an anode, a cathode, a control grid, and a second grid; atransformer having a current winding consisting of relatively large wireand a potential winding consisting of relatively small wire wound turnby turn in the same sense along and with corresponding turns of at leasta part of said current winding to form a twin winding therewith;,aconnection from said cathode to said anode through a source of directcurrent potential and through said potential winding; a second tubehaving an anode a cathode, a control grid, and a second grid;connections whereby said current Winding of said transformer isassociated with said control grid and cathode of said second tube; andmeans for applying a bias to said second grid.

In testimony whereof, I afiix my signature. FREDERICK H. DRAKE.

